In the studies described in this progress report, we have investigated the nutritional and hormonal control of the biosynthesis of fatty acid synthetase and have also carried out mechanistic studies on fatty acid synthetase complex. Starvation and experimental diabetes reduce the activities of liver acetyl CoA carboxylase and fatty acid synthetase 30-50 fold. Refeeding of the starved animals and insulin administration restore the activities of these enzymes to the normal levels. Long-term feeding of high fructose diet (60% fructose by weight) also enhances these activities to values 2-3 fold higher than those obtainable in normal animals alone. We have also carried out immunological and catalytic cross-reactivity studies of fatty acid synthetases from avian and mammalian livers. Avian enzymes cross-react and cross-inhibit the activity for fatty acid synthesis and show little or no reactivity against the mammalian enzyme. Furthermore, the antigen-antibody complex catalyses all the partial reactions of fatty acid synthesis except the condensation-CO2 exchange reaction.